This invention relates to the preparation of melamine-urea-formaldehyde resins modified with cyclic urea prepolymer and sodium metabisulfite, to their use in binders, and to composite products prepared using the binders. Products prepared with the binders include wood composites such as particleboard, medium density fiberboard, plywood and fiber composites such as glass mats and other fiber composites
Urea-formaldehyde resins are used in the preparation of wood composites such as particleboard and medium density fiberboard because of their processing advantages and low cost relative to other typical wood adhesives. Urea-formaldehyde resins are typically prepared by reacting urea and formaldehyde to form various methylolated ureas and their condensation products in ratios that depend on the temperature, pH and time for the reaction.
The use of urea-formaldehyde resins is limited to applications where exterior durability is not required. Melamine has been included in urea-formaldehyde binders to improve the moisture resistance of composites made with these binders. The moisture resistance of wood composites is measured by various test methods. Most commonly, the thickness swell and water absorption after 24 hours according to ASTM D 1037 are measured. Additionally, other international test methods are also being followed in North America. For example, the bending strength or modulus of rupture (MOR) can be measured after a soaking period following the Japanese standard and test method JIS A 5908. Also, the durability of the composites can be evaluated by measuring the thickness swell and internal bond strength of the composites after a strenuous 21-day cyclic test including soaking, freezing and baking cycles according to the European test method EN 321 (formerly known as V 313). The melamine can be incorporated into wood composites in the form of a melamine-urea-formaldehyde resin, a melamine-formaldehyde resin, or as a blend of a urea-formaldehyde resin and melamine-formaldehyde resin. U.S. Pat. No. 3,827,995 and 3,979,492) However, the addition of melamine adds significant cost to the composite.
Urea-formaldehyde binders are also used in other industrial applications, such as, for instance as binders for glass fibers used to make mats that are used in roofing shingles, carpet, vinyl flooring, and various other composites. In the manufacture of glass mat, the tensile strength of hot-wet samples of mat is routinely measured and compared to the tensile strength of dry samples to calculate a hot-wet tensile retention. Therefore, it is advantageous to modify the urea-formaldehyde binders with substances such as melamine or latexes that increase the moisture resistance of the cured mats.
Further, when urea-formaldehyde resins are used as a binder component they release formaldehyde into the environment during cure. Formaldehyde also can be released subsequently from the cured resin, particularly when the cured resin is exposed to acidic environments. Such formaldehyde release is undesirable, particularly in enclosed environments. In such environments, formaldehyde is inhaled by workers and comes into contact with the eyes, the mouth, and other parts of the body. Formaldehyde is malodorous and is thought to contribute to human and animal illness.
Various techniques have been used to reduce formaldehyde emission from urea-formaldehyde resins. Use of formaldehyde scavengers and various methods for resin formulation, including addition of urea as a reactant late in the resin formation reaction, are techniques often used to reduce formaldehyde emission. However, use of formaldehyde scavengers often is undesirable, not only because of the additional cost, but also because it affects the characteristics, or properties, of the resin. For example, using ammonia as a formaldehyde scavenger often reduces the resistance of the cured resin to hydrolysis (degradation). Later addition of urea to reduce free formaldehyde concentration in the resin generally yields a resin that must be cured at a relatively low rate to avoid smoking in certain processes or that cures more slowly than resins without scavengers in other processes. Resin stability also can be adversely affected by such treatments. Melamine has also been used to prepare resins which have low formaldehyde emissions (U.S. Pat. No. 5,681,917).
Melamine resins, particularly melamine-urea-formaldehyde (MUF resins), while useful to make moisture resistance particleboard, have very poor dilutability in water and therefore are inconvenient to handle at particleboard and MDF mills. The use of melamine resins as binders for glass mat or wet-process hardboard is severely limited by the lack of water solubility or dilutability of typical melamine-urea-formaldehyde binders.
It is known in the art that MF and MUF resins can be sulfonated to achieve water solubility (see, for example, U.S. Pat. No. 5,710,239 and EP 913412). However, when sulfonation is used to achieve water dilutability, an inverse relationship occurs between water dilutability in the liquid resin and the moisture resistance properties of the cured composite. Only a moderate amount of sulfonation can be tolerated if moisture resistance is to be maintained in composite materials such as particleboard and MDF, and this moderate level of sulfonation yields a moderately low water dilutability. Typical sulfonated MUFs currently manufactured for use as wood binders have a water dilutability of 2:1 (two parts of water to one part of resin) before the resin comes out of solution.
MUF resins can be made with a cyclic urea-formaldehyde prepolymer as disclosed in copending U.S. patent application Ser. No. 09/339,030, which is hereby incorporated by reference in its entirety. This patent discloses the use of the cyclic urea-formaldehyde prepolymer as a partial replacement for melamine while achieving comparable moisture resistance properties. However, the water dilutability of materials made following the process described in this patent is only 2:1 or lower, and after the water dilutability is exceeded, the resins may become very sticky and difficult to clean up.
The present invention is directed to a binder comprising a melamine-urea-formaldehyde (MUF) resin modified with a cyclic urea prepolymer and with sodium metabisulfite.
MUF resins prepared with cyclic urea prepolymer and sodium metabisulfite display superior properties beyond what would have expected based on combining the known cyclic urea technology and known sulfonation technologies. Specifically, whereas the MUFs made according to the prior art processes have relatively low water dilutability, MUFs modified with both the cyclic urea-formaldehyde prepolymer and sodium metabisulfite exhibit a water dilutability in excess of 20:1 without the formation of a precipitate. This unique advantage makes the handling of these resins more like conventional UF resins, which can be rinsed off with water or diluted to low concentrations to improve resin distribution when applied to a substrate, such as in blowline application on fiber in the preparation of MDF. In wet processes such as glass mat the improvement in dilutability allows the modified MUF resins to run at much higher melamine levels than what conventional MUF technology can allow.
Moreover, other properties of the binder, such as press time and thickness swell, are not compromised by the addition of the sodium metabisulfite.
The present invention is further directed to composite products prepared with a substrate and a binder as defined above. In particular, the present invention is directed to wood based composites such as particleboard and medium density fiberboard. The invention is also directed to composites such as glass mat and other glass fiber based composites generally referred to as fiber reinforced plastics.
Despite the great improvement in water dilutability for the modified MUF resins versus conventional MUF resins, wood composites made with the modified MUF resins display better moisture resistance properties than those made with conventional MUF resins of the same melamine level. For example, particleboard and MDF made with the modified MUFs at a given melamine level are sufficiently superior in thickness swell, water absorption, wet MOR and performance in the V313 test to allow the board-manufacturers to reduce resin application rates while maintaining the same properties that are achieved at higher resin applications rates with conventional MUFs. Alternatively, the melamine level of the MUF resins can be reduced, and the thickness swell, water absorption, wet MOR and performance in the V313 test for the particleboard and MDF are the same as if the boards had been made with a higher melamine content conventional MUF.
In glass mat, the modified MUF resins yield superior moisture resistance to UF resins as indicated by better hot-wet tensile strength retentions.